The present invention relates to electrochemical fuel cell plates. In particular, the invention provides an electrochemical solid polymer fuel cell plate with improved reactant man folding and sealing in a fuel cell stack.
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes generally each comprise a porous, electrically conductive sheet material and an electro catalyst disposed at the interface between the electrolyte and the electrode layers to induce the desired electrochemical reactions. The location of the electro catalyst generally defines the electrochemically active area.
Solid polymer fuel cells typically employ a membrane electrode assembly (MEA) consisting of an ion-exchange membrane as electrolyte disposed between two electrode layers. The membrane, in addition to being ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant streams from each other.
The MEA is typically interposed between two separator plates which are substantially impermeable to the reactant fluid streams. The plates act as current collectors and provide support for the MEA. Surfaces of the separator plates which contact an electrode are referred to as active surfaces. The separator plates can have grooves or open-faced channels formed in one or both surfaces thereof, to direct the fuel and oxidant to the respective contacting electrode layers, namely, the anode on the fuel side and the cathode on the oxidant side. Such separator plates are known as flow field plates, with the channels, which can be continuous or discontinuous between the reactant inlet and outlet, being referred to as flow field channels. The flow field channels assist in the distribution of the reactant across the electrochemically active area of the contacted porous electrode. In some solid polymer fuel cells, flow field channels are not provided in the active surfaces of the separator plates, but the reactants are directed through passages in the porous electrode layer. Such passages may, for example, include channels or grooves formed in the porous electrode layer or can just be the interconnected pores or interstices of the porous material.
In a fuel cell stack, a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, an active surface of the separator plate faces and contacts an electrode and a non-active surface of the plate can face a non-active surface of an adjoining plate. In some cases, the adjoining non-active separator plates can be bonded together to from a laminated plate. Alternatively, both surfaces of a separator plate can be active. For example, in series arrangements, one side of a plate can serve as an anode plate for one cell and the other side of the plate can serve as a cathode plate for the adjacent cell, with the separator plate functioning as a bipolar plate. Such a bipolar plate can have flow field channels formed on both active surfaces.
The fuel stream which is supplied to the anode separator plate typically comprises hydrogen. For example, the fuel stream can be a gas such as a substantially pure hydrogen or a reformat stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol can be used. The oxidant stream, which is supplied to the cathode separator plate, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
A fuel cell stack typically includes inlet ports and supply manifolds for directing the fuel and the oxidant to the plurality of anodes and cathodes respectively. The stack often also includes an inlet port and manifold for directing a coolant fluid to interior passages within the stack to absorb heat generated by the exothermic reaction in the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unrelated fuel and oxidant gases, as well as an exhaust manifold and outlet port for the coolant stream exiting the stack. The stack manifolds, for example, can be internal manifolds, which extend through aligned openings formed in the separator layers and Mesa, or can comprise external or edge manifolds, attached to the edges of the separator layers.
Conventional fuel cell stacks are sealed to prevent leaks and inter-mixing of the fuel and oxidant streams. Fuel cell stacks typically employ fluid tight resilient seals, such as electrometric gaskets between the separator plates and membranes. Such seals typically circumscribe the manifolds and the electrochemically active area. Sealing is effected by applying a compressive force to the resilient gasket seals.
The passageways which fluidly connect each electrode to the appropriate stack supply and/or exhaust manifolds typically comprise one or more open-faced fluid channels formed in the active surface of the separator plate, extending from a reactant manifold to the area of the plate which corresponds to the electrochemically active area of the contacted electrode. In this way, for a flow field plate, fabrication is simplified by forming the fluid supply and exhaust channels on the same face of the plate as the flow field channels. However, such channels can present a problem for the resilient seal which is intended to fluidly isolate the other electrode (on the opposite side of the ion exchange membrane) from this manifold. Where a seal on the other side of the membrane crosses over open-faced channels extending from the manifold, a supporting surface is desirable or required to bolster the seal and to prevent the seal from leaking and/or sagging into the open-faced channel. One solution adopted in conventional separator plates is to insert a bridge member which spans the open-faced channels underneath the resilient seal. The bridge member preferably provides a sealing surface which is flush with the sealing surface of the separator plate so that a gasket-type seal on the other side of the membrane is substantially uniform compressed to provide a fluid tight seal. The bridge member also prevents the gasket-type seal from sagging into the open-faced channel and restricting the fluid flow between the manifold and the electrode. Instead of bridge members, it is also known to use metal tubes or other equivalent devices for providing a continuous sealing surface around the electrochemically active area of the electrodes (see, for example, U.S. Pat. No. 5,750,281), whereby passageways, which fluidly interconnect each electrode to the appropriate stack supply or exhaust manifolds, extend laterally within the thickness of a separator or flow field plate, substantially parallel to its major surfaces.
Conventional bridge members are affixed to the separator plates after the plates have been milled or molded to form the open-faced fluid channels. One problem with this solution is that separate bridge members add to the number of separate fuel cell components which are needed in a fuel cell stack. Further, the bridge members are typically bonded to the separator plates, so care should be exercised to ensure that the relatively small bridge members are accurately installed and that the bonding agent does not obscure the manifold port. It is also preferable to ensure that the bridge members are installed substantially flush with the sealing surface of the separator plate. Accordingly, the installation of conventional bridge members on separator plates adds significantly to the fabrication time and cost for manufacturing separator plates for fuel cell assemblies. Therefore, it is desirable to obviate the need for such bridge members, and to design an electrochemical fuel cell stack so that the fluid reactant streams are not directed between the separator plates and MEA seals.
In the present approach, passageways fluidly interconnecting an anode to a fuel manifold and interconnecting a cathode to an oxidant manifold in an electrochemical fuel cell stack are formed between the non-active surfaces of a pair of adjoining separator plates. The passageways then extend through one or more ports penetrating the thickness of one of the plates thereby fluidly connecting the manifold to the opposite active surface of that plate, and the contacted electrode. Thus, the non-active surfaces of adjoining separator plates in a fuel cell stack can cooperate to provide passageways for directing both reactants from respective fuel and oxidant manifolds to the appropriate electrodes. Of course, the fuel and oxidant reactant streams are fluidly isolated from each other, even though they are directed between adjoining non-active surfaces of the same pair of plates. Coolant passages can also be conveniently provided between non-active surfaces of adjoining separator plates.
An electrochemical fuel cell stack with improved reactant man folding and sealing comprises:
(a) a plurality of membrane electrode assemblies each comprising an anode, a cathode, and an ion-exchange membrane interposed between the anode and cathode;
(b) a pair of separator plates interposed between adjacent pairs of the plurality of membrane electrode assemblies, the pair of separator plates comprising:
an anode plate having an active surface contacting an anode, and an oppositely facing non-active surface, and
a cathode plate having an active surface contacting a cathode, and an oppositely facing non-active surface which adjoins the non-active surface of the anode plate; and
(c) a fuel supply manifold for directing a fuel stream to one, or preferably more of the anodes, and an oxidant supply manifold for directing an oxidant stream to one, or preferably more, of the cathodes, and fuel and oxidant stream passageways fluidly connecting the fuel and oxidant supply manifolds to an anode and a cathode, respectively,
wherein at least one of the fuel and oxidant stream passageways traverses a portion of the adjoining non-active surfaces of a pair of the separator plates.
The electrochemical fuel cell stack can optionally further comprise an oxidant exhaust manifold for directing an oxidant stream from one, or preferably more, of the cathodes, and/or a fuel exhaust manifold for directing a fuel stream from one, or preferably more, of the anodes. In preferred embodiments, reactant stream passageways fluidly interconnecting the reactant exhaust manifold to the electrodes also traverse a portion of adjoining non-active surfaces of a pair of the separator plates.
In further embodiments, passages for a coolant can also be formed between cooperating non-active surfaces of adjoining anode and cathode plates, or one or more coolant channels can be formed in the active surface of at least one of the cathode and/or the anode separator plates. In an operating stack, a coolant can be actively directed through the cooling channels or passages by a pump or fan, or alternatively, the ambient environment can passively absorb the heat generated by the electrochemical reaction within the fuel cell stack.
The separator plates can be flow field plates wherein the active surfaces have reactant flow field channels formed therein, for distributing reactant streams from the supply manifolds across at least a portion of the contacted electrodes.
In the present approach, passageways for both the fuel and oxidant reactant streams extend between adjoining non-active surfaces of the same pair of plates, but the passageways are fluidly isolated from each other. To improve the sealing around the reactant stream passageways located between adjoining non-active surfaces of the separator plates, the fuel cell stack can further comprise one or more gasket seals interposed between the adjoining non-active surfaces. Alternatively, or in addition to employing gasket seals, adjoining separator plates can be adhesively bonded together. To improve the electrical conductivity between the adjoining plates, the adhesive is preferably electrically conductive. Other known methods of bonding and sealing the adjoining separator plates can be employed.
In the embodiments of an electrochemical fuel cell stack described above, the manifolds can be selected from various types of stack manifolds, for example internal manifolds comprising aligned openings formed in the stacked membrane electrode assemblies and separator plates, or external manifolds extending from an external edge face of the fuel cell stack.
As used herein, adjoining components are components which are in contact with one another, but are not necessarily bonded or adhered to one another. Thus the terms adjoin and contact are intended to be synonymous.